Enhanced electrical properties in sub-10-nm WO3 nanoflakes prepared via a two-step sol-gel-exfoliation method
Serge Zhuiykov
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Eugene Kats
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Materials Science and Engineering Division
, CSIRO, 37 Graham Road, Highett,
VIC 3190, Australia
The morphology and electrical properties of orthorhombic -WO3 nanoflakes with thickness of ~7 to 9 nm were investigated at the nanoscale with a combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), current sensing force spectroscopy atomic force microscopy (CSFS-AFM, or PeakForce TUNA), Fourier transform infra-red absorption spectroscopy (FTIR), linear sweep voltammetry (LSV) and Raman spectroscopy techniques. CSFS-AFM analysis established good correlation between the topography of the developed nanostructures and various features of WO3 nanoflakes synthesized via a two-step sol-gel-exfoliation method. It was determined that -WO3 nanoflakes annealed at 550C possess distinguished and exceptional thickness-dependent properties in comparison with the bulk, micro and nanostructured WO3 synthesized at alternative temperatures.
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Background
The layered transitional quasi-two-dimensional (Q2D)
semiconductor oxides MO3 (M = Mo, W), have recently
attracted significant interest because they demonstrate
quantum confinement effects at the few-layer limit [1,2].
Among them, tungsten trioxide (WO3) is an n-type
semiconductor in an indirect bandgap of 2.6 to 2.9 eV [3] with
excellent electrochromic and gasochromic properties [4].
It has electron Hall mobility of ~12 cm2V1 s1 at room
temperature and responsive to the blue end of the visible
spectrum ( < 470 nm) [5]. Extrinsic n-doping is therefore
not required for WO3 to exhibit significant conductivity.
Similar to graphene, WO3 can be mechanically or
chemically exfoliated to provide fundamental layers. However,
unlike graphene, which does not have bandgap, Q2D
WO3 has rather large bandgap, making Q2D WO3
nanoflakes more versatile as candidates for thin, flexible devices
and potential applications in catalysis [6], optical switches
[7] displays and smart windows [8], solar cells [9] optical
recording devices [10] and various gas sensors [11]. It
has become one of the most investigated functional
semiconductor metal oxides impacting many research
fields ranging from condensed-matter physics to
solidstate chemistry [10].
However, despite great interest of the research and
industrial communities to the bulk and
microstructured WO3, nanoscaled Q2D WO3 with thickness less
than ~10 nm has received relatively little attention so
far compared to its microstructured counterparts and
to Q2D transitional metal dichalcogenides MX2 (M = Mo,
W; X = S, Se, Te). In addition, last year's reports on
alternative transitional semiconductor oxide Q2D MoO3 have
exhibited exceptional thickness-dependent properties and
the substantial increased of the charge carriers mobility
(up to 1,100 cm2 V1 s1) in Q2D MoO3 [2,12]. It was also
recently proven for MoSe2 that the reduction of bandgap
can be achieved through decreasing the thickness of Q2D
nanoflakes down to monolayer [13]. Therefore, realization
of WO3 in its Q2D form can further engineer the
materials' electrical properties, as quantum confinement effects
in 2D form will significantly influence charge transport,
electronic band structure and electrochemical
properties [3]. More importantly, nanostructuring of WO3 can
enhance the performance of this functional Q2D material
revealing unique properties that do not exist in its bulk
form [2].
The development of Q2D materials is generally a
twostep process, the synthesis of the layered bulk material
followed by the exfoliation process [14]. Although there is
a wide range of controlled methods of synthesis available
to produce different morphologies of WO3 nanostructures,
such as microwave-assisted hydrothermal [15],
vapourphase deposition [16], sol-gel [17], electron-beam [18] and
arc-discharge [19], synthesis of Q2D WO3 is a topic that
is yet to be widely explored. For instance, in a recent
report, it was demonstrated that one possible way of
bandgap reduction in bulk WO3 is to increase its sintering
temperature [20]. However, what is the most favourable
sintering temperature for exfoliation Q2D WO3
nanoflakes remains largely unexplored.
In this work, we present for the first time new
distinguishing thickness-dependent electrical properties
of Q2D -WO3 obtained for nanoflakes with thickness
below ~10 nm developed via two-step sol-gel-exfoliation
method. These properties were mapped without damaging
the sample by carefully controlling the sample-tip force.
This is performed by using current sensing force
spectroscopy atomic force microscopy (CSFS-AFM), also known
as PeakForce TUNA [21], which allowed simultaneous
measurements of the topography and the current flowing
between the tip and the sample from the real-time analysis
of force-distance curves measured for a tip oscillating in
the kilohertz regime, far below the resonance frequency of
the cantilever [22]. This technique also provided direct
control of the force ap (...truncated)